The determination of substances for diagnostic or scientific purposes in liquid test samples by means of agglutination and the inhibition of agglutination of a sentized solid phase, such as red blood cells, colloidal particles, latex or the like is known for more than 20 years. The test proceeds as follows: the solid carrier phase is activated by a binding agent (e.g. tannic acid, glutaraldehyde, difluoro-dinitro benzene) and sensitized by an antigen or an antibody. It may also spontaneously adsorb the sensitizing substance.
The sensitized solid phase is mixed with the test liquid containing the soluble antigen or antibody to be determined and with a determined amount of a specific antibody or antigen with which the sensitized solid phase reacts in binding reaction. In the absence of foreign soluble antigen in the test liquid, an antigen-sensitized solid phase will be agglutinated by the specific antibody. In the presence of foreign soluble antigen, the latter will inhibit agglutination of the solid phase. Conversely, an antibody-sensitized solid phase will be agglutinated in the presence of a determined amount of specific antigen absent any foreign antigen and this agglutination will be inhibited when foreign soluble antibody is also present in the reacting medium from the test liquid.
This system, commonly referred to as the hemagglutination inhibition test (HAI), is relatively sensitive, but has lately been superseded by more refined techniques amount which radio-immuno-assay is outstanding. Radio-immuno-assays, however, require radioelements, expensive instrumentation and skilled labor, and it would be desirable to improve the HAI tests and increase their sensitivity, so that they can accomplish the same function as radio-immuno-assays at a much reduced cost.